Powder metallurgy has been used to make electric contact materials. Refractory metals, by which is meant metals having a melting temperature above 1600.degree.C, in powder form, may be molded into a compact which is sintered to form a porous matrix and which is infiltrated by molten metal having a composition intended to provide the desired properties. The resulting product is then used to make an electric contact.
The impregnating metal must have a lower melting temperature than that of the sintered matrix to avoid destruction of the latter during the infiltration step.
When intended for the contacts of vacuum switches, such a material must meet stringent requirements such as freedom from gas content, reliable operation while carrying large currents, such as 25 KA and higher, and low breakoff currents of less than 5A, an adequately low welding force such as less than 500N, and others. Resistance to destruction by burning must be sufficiently high, such switches being required to have a service life of more than 10,000 switching cycles under nominal current conditions, and approximately fifty direct short-circuit openings.
The prior art, exemplified by German published patent application No. 1,640,039, has proposed the use of chromium or cobalt for the sintered matrix, and which is impregnated with copper or silver. Chromium powder has the disadvantage that it is difficult to mold into a dimensionally stable compact suitable for sintering, even by molding under very high pressure. Cobalt has the disadvantage that because of its ductility powder particles of this metal deform under the pressure required to form it to a compact for sintering, resulting in a matrix having closed pores which cannot be satisfactorily infiltrated by the molten metal impregnant.
However, the above type of material has the advantage that the matrix provides good resistance to burn-off during contact operations under high electric currents, while the impregnant provides high electric conductivity. In fact, the burn-off involved is less than that which can be provided by either the matrix metal or the impregnating metal when used alone.
To obtain this advantage, it is necessary that the matrix retains its as-sintered physical structure after infiltration by the high conductivity impregnant. This is complicated by the fact that a relatively large pore matrix is desirable, such as obtained by using metal powder having a particle size of up to 150 microns, to facilitate the infiltration of the impregnating metal. This introduces the problem that the matrix may include poorly interbonded powder particles having relatively few and weak interbonding portions after the sintering, and if at the impregnating temperature there exists substantial solubility between the matrix metal and the impregnating metal, such interbonding portions may be dissolved during the infiltration of the impregnant with the result that the matrix powder particles of chromium or cobalt may appear as isolated or unbonded powder particles in the finished material.